1. Field of the Invention
This invention relates to waterborne phenoxy resins which are blended with modifier resins to provide significant improvements in key properties.
2. Description of the Prior Art
Phenoxy resins, sometimes referred to as "thermoplastic polyhydroxyethers," are known in the art as components of corrosion-resistant coatings, e.g., zinc-rich coatings for metallic substrates, as described in U.S. Pat. Nos. 4,370,382, 4,391,855 and 4,476,260. Phenoxy-based coatings have not, however, proven to be entirely satisfactory.
More specifically, prior phenoxy-based coatings have not provided satisfactory adhesion to smooth substrates, e.g., metallic substrates, and have been subject to degradation by corrosion, particularly in high humidity environments. To obviate such problems, U.S. Pat. No. 4,638,038 discloses a phenoxy resin having pendant secondary hydroxyl groups of which about 3 to about 50 percent of such hydroxyl groups have been reacted to produce moieties having pendant carboxyl groups. Such hydroxyl-reacted carboxylic acid-containing phenoxy resins can be used as general purpose and corrosion-resistant coatings, useful in water-dispersable coatings, adhesives and laminating systems.
However, it would be desirable to provide a resin system that would offer even further improved performance. As an example, the hydroxyl-reacted, carboxylic acid-based phenoxy resins generally produce milky-white dispersions in water with relatively limited shelf stability. Additionally, it would be desirable to be capable of offering more flexible coatings having lower viscosities, providing the potentiality for much higher solids contents.
Phenoxy resin blends with relatively softer resins have been found useful in making various injection molded products. Blends of phenoxy resins with relatively softer resins are thus described in U.S. Pat. Nos. 3,925,504 and 4,337,330.
While such coatings provide excellent corrosion resistance on non-deformed surfaces, these coatings are often not fully satisfactory for uses in which the substrate and coating undergo substantial deformation, or where bake temperatures of less than about 400.degree. F. are used (such uses often being termed "low bake" systems). As illustrative examples of such uses, there can be mentioned the stamping and forming of sheet metal, as in the automobile manufacturing industry. As a result of such deformation, corrosion resistance can be seriously degraded.
European published application No. 86113893.1 (equivalent to co-pending U.S. application Ser. No. 850,783) discloses a formable coating composition comprising a phenoxy resin and about 1 to about 50 percent by weight of the total resin of a modifier resin which is relatively soft in comparison to the phenoxy resin, is compatible with the phenoxy resin, and has specific reduced viscosity, glass transition temperature, solubility and molecular weight characteristics. By the term "formable compositions" it is meant that the coating is able to withstand physical operations to the substrate (e.g., rolling, bending, stamping, cutting, etc.) without significant damage to the continuity and adhesion of the coating. Accordingly, the protective properties of the coating are preserved.
Waterborne phenoxy resins are known (U.S. Pat. No. 4,355,122), as are coatings made therefrom (U.S. Pat. No. 4,374,875). These resins are listed in Title 175.300 of Chapter 21, CRF, of the U.S. Food and Drug Administration.
In view of the general trend toward waterborne systems within the coatings industry, it would be desirable to provide a waterborne phenoxy resin system which provides an adequately flexible coating, even when crosslinked. Further, it would be desirable to provide such a waterborne phenoxy resin system which is characterized by a relatively low viscosity in comparison to phenoxy resin coatings so as to provide the potential for a much higher resin or solids content. It would additionally be desirable to provide systems of this sort which permit lower bake temperatures to form coatings which are characterized by further improved corrosion resistance. Importantly, it would be particularly desirable to provide an improved phenoxy resin system which remains stable upon storage and does not gel upon such storage for long periods of time and which likewise remains stable even when pigmented.
Significant improvements to certain water-borne phenoxy systems are described in co-pending U.S. application Ser. No. 078,277, filed July 27, 1987 now abandoned. That application provides a waterborne coating composition comprising a blend of phenoxy resin and from about 5 to about 50 percent, by weight of the total resin, of a modifier resin, compatible with the phenoxy resin, and being relatively soft in comparison thereto, and having:
(1) a reduced viscosity of about 0.1 to about 2, preferably about 0.2 to about 1, dL/g in tetrahydrofuran at 25.degree. C.;
(2) a glass transition temperature (Tg) of about -120 to about 30, preferably about -100 to about 0.degree. C.; and
(3) a number average molecular weight of about 500 to about 10,000, most typically from about 500 to 3,000.
Both the phenoxy and the modifier resin must contain hydroxyl groups, and about 3 to about 50 percent of the total hydroxyl groups present are reacted to produce moieties having carboxyl groups, which are then neutralized to render the blend waterborne.
In creating products for the coatings industry, it is imperative to address the regulations of the EPA of the U.S. Government relating to Volatile Organic Content ("VOC"). Although the prior art systems described above are capable of producing room-temperature-filming, highly-flexibilized phenoxy and epoxy coatings, they are produced via solvent processes and their coating processes usually involve high levels of coalescing solvents, and therefore they significantly exceed current VOC standards for compliance. In general, the lower the bake temperature, the higher the VOC because more coalescing solvent is needed to film the resin. On the other hand, at low levels of coalescing solvent, clear films can not ordinarily be produced by drying at low (e.g., room) temperatures.